Canola rapeseed cultivars low in erucic acid and glucosinolates, have become the most important oilseed crop in Canada, and are an important source of edible oil throughout the world. The oil, which constitutes about 40% of the seed, is used for a variety of food products including salad oils, cooking oils and hydrogenated products such as margarines and shortenings. In 1988, oil derived from canola accounted for approximately 56% of Canada's vegetable oil supply. It ranked third among the world's oilseed crops, and was second only to wheat in Canada in terms of farm value and area planted. While not presently used for human consumption, canola flour is a very good source of protein, containing an excellent balance of essential amino acids. Canola meal contributes up to 30% of the animal feed protein used in Canada. This protein, which is about 27% of the canola seed, has performed very well experimentally in combination with other foods, such as ground meat, baking flour, and egg albumen. Canola, or "Low Erucic Acid Rapeseed" (LEAR) oil received "Generally Recognized as Safe" (GRAS) status in the United States on 28 January 1985.
Present methods for the extraction of oil from canola involve mechanical pressure to directly press oil from the seed, and/or direct solvent extraction using hexane. The most frequently employed process uses a combination of these two methods, and is referred to as prepress-solvent extraction (PSE). In this process, clean seed is crushed or flaked to rupture the oil-bearing cells. The flaking of the seeds also allows them to be heated for 30-40 minutes in a series of cooking kettles (stack cooker) used in the cooking operation which follows. Cooking at 80-100.degree. C. for about 30 minutes is done for two principal reasons: (1) it is believed that heat treatment conditions the cell walls so as to allow more rapid extraction of the oil, increasing the permeability of the seed to the hot miscella (oil+hexane); and (2) it destroys seedborne enzymes, and in particular myrosinases, which catalyze the breakdown of glucosinolates into the deleterious compounds isothiocyanates, nitriles, oxazolidinethiones, and sulfate and thereby yielding crude oil high in sulfur. After the flaking and cooking operations, the seed meal is passed to an expeller or screw press which squeezes about 60% of the seed oil from the meal, and the residue is referred to as "oil cake". This cake is subsequently ground and conveyed to a solvent extractor, leaving the resulting meal in a solvent-saturated condition. Solvent is recovered from both the meal and miscella by toasting and distillation, leaving a crude oil for further refinement and a meal containing approximately 33% hull material.
The current processing method suffers several drawbacks, these being related to the quality of the oil and meal and products. The hull material in the meal fraction contributes little or no nutrition, and is a problem in the feeding of monogastric animals and poultry because it limits the quantity of meal that can be used in feed formulations, due to the excessive fiber content. The black hull material in the meal makes it visually undesirable as a potential ingredient in foods for human consumption and for pet foods.
The heat treatment (cooking operation; 80-100.degree. C.) has a deleterious effect on both the oil and meal. Protein in the meal is extensively denatured, essential amino acids (e.g. lysine) are partially destroyed, and certain physical properties of the protein (e.g. color, water solubility) important in food formulations, are adversely affected. Dark and stable melanine-like compounds are produced under heat conditions from the interaction of phenols, carbohydrates and protein. Oil extracted from the heat treated, flaked seed is considerably darker than that obtained from cold-pressed seed, and is of appreciably lower quality, especially where green seed is involved.
Several patents have described certain oilseed processing involving liquid cyclones.
U.S. Pat. No. 3,615,657 (E.A. Gastrock et al), is specific for the preparation of protein concentrates low in gossypol from cottonseed. The gossypol is contained in pigment glands which if ruptured release the gossypol thereby permitting it to combine with lysine in the protein. The patent describes procedures adopted to retain the pigment glands intact and subsequently after releasing them from the matrix by a wet milling process in hexane, separating them from the protein containing particles by a liquid cyclone process.
U.S. Pat. No. 3,972,861 (H.A. Gardner et al) closely resembles Gastrock et al where the basic problem exists of retaining the pigment glands intact and their separation in a liquid cyclone is undertaken. Prior drying of the meats and method of comminution of the meats are both critical factors in the Gastrock et al patent.
U.S Pat. No. 4,146,534 (D.J. Armstrong) is based on the premise that the protein in vegetative protein sources (oilseeds) is located in discrete particles called protein bodies possessing a smooth texture and being mostly smaller than 10 microns in size. The strategy is to break up the tissue fine enough so that the protein bodies are released from the vegetative debris. The finely ground vegetative material may be subjected to a preliminary separation by air classification and the finest particle fraction preferably 60% of which has particle sizes of 10 microns or less is then subjected to a separation in hydrocyclones after suspension in water or aqueous-alcohol having a low protein solubility. The protein bodies are concentrated in the underflow fraction and may be further subjected to a cylcone separation or recovered directly. The overflow fraction is denuded of protein bodies and enriched with vegetative debris. The overall effect of the cyclones is to yield a protein enriched fraction and a protein denuded fraction.
Canadian Patent 1,089,849, Nov. 18, 1980, F.W. Sosulski et al describes fractionating rapeseed (or mustard) by steps which can include a liquid cyclone separation of seed meal flour from hulls.
Some effort has been made to solvent extract oilseeds in a contained system.
U.S Pat. No. 4,453,832 (H.O. Schumacher et al) describes the apparatus used in a system for mixing solid material to be extracted with an extractant with provision to limit the access of atmospheric oxygen to the process, and with containment of the extractant (solvent) vapors from reaching the atmosphere. The apparatus is particularly suitable for the extraction of oilseeds when using a solvent such as hexane. This apparatus does not operate as a cyclone system and does not use means to separate hulls from the meal or from the meats solids.
None of the prior processes have provided a continuous flow process from intact seed through to final oil, flour and seed coat meal under non-oxidizing conditions.
In summary, many desirable improvements over the present processing methods for the production of oil-seed (especially canola) meal and oil have been identified: (1) reduction of seed coat (hull) material in oilseed meal to upgrade the protein content and extend its utilization potential (i.e. to humans and monogastric animals); (2) reduction in the loss of nutritional factors in the meal and improvement in meal quality; (3) improved oil quality; (4) removal of antinutritional factors from meal and oil; (5) improved plant safety; (6) reduction of hexane loss from process equipment; and (7) the operation is achieved at ambient temperature. Other potential improvements to the current technology include: (8) markedly reduced process time, thereby reducing the possibility of interaction between seed components (e.g. polysaccharide/amino acid interactions; melanine compounds); (9) increased flexibility in terms of plant expansion or contraction; (10) ability to operate in a fully continuous mode; (11) reduction in the number of unit operations required to produce high quality product; and (12) simplification of the equipment, especially that used in extraction.
We have observed that when rapeseed (canola) is subjected to particle size reduction in liquids, the seed coat (hull) resists pulverization, and this factor can be used to assist separation of the seed coat from the seed endosperm which readily pulverizes under these conditions. This led us to consider continuous whole oilseed processing in a liquid system.